Bonding apparatus, such as those used in the semiconductor assembly industry, commonly rely on ultrasonic energy for bonding electrically conductive wire to electrical contact pads.
FIG. 1 is a side view of a conventional ultrasonic transducer 100 of the prior art that is now widely available in the market. The conventional ultrasonic transducer 100 comprises a horn 102 with a substantially circular cross-section on which is located a mounting barrel 104 consisting of a hollow cylinder for mounting the transducer 100. Other conventional ultrasonic transducers may comprise horns that have generally rectangular cross-sections. Ultrasonic energy is produced by driving means, which may comprise an ultrasonic driver 108 coupled to the horn by securing it between a front plate 110 and a back plate 112 of the transducer 100. The front plate 110, ultrasonic driver 108 and back plate 112 may be compressed together using an axial bolt (not shown) under high torque. A bonding tool, such as a capillary 106, is attached to the tip of the horn 102 at an opposite end of the horn 102 away from the location of the ultrasonic driver 108.
During operation, a set of piezoelectric discs comprised in the ultrasonic driver 108 is subjected to an alternating electrical signal which causes the discs to expand and contract according to the signal. Consequently, an ultrasonic wave is generated at the applied frequency of the electrical signal. Since the wave produced has a small amplitude, an amplifying device (in this case the horn 102) is required before the wave is passed to the bonding tool such as the capillary 106.
The horn 102 is usually constructed with a decreasing cross-sectional area along the axis of the transducer 100 towards the capillary 106. The ultrasonic energy that is transmitted causes the capillary 106 to oscillate parallel to the longitudinal axis of the horn 102. The ultrasonic energy at the tip of the capillary 106 is applied to wire that is fed through the capillary 106 at the bonding site whereby to weld the wire to a bonding surface at the bonding site during wire bonding.
As noted above, the horn 102 of such a conventional ultrasonic transducer 100 has a generally circular cross-section throughout. During bonding, the horn 102 is subjected to bending loads as the transducer 100 is lifted up and down during the bonding process. Moreover the transducer 100 undergoes continuous resonance during operation. It is hence held at node locations by using a thin web of material that connects the horn 102 to the barrel 104 at a node location of a standing ultrasonic wave within the horn 102 generated by the activated ultrasonic driver 108. Generally, the transducer 100 is held on a bonding system by clamping it on the barrel 104 such that strain is transferred to the horn and the ultrasonic driver. As a result, considerable strain is caused on the horn 102 during operation.
While horns with circular or rectangular cross-sections are generally adequate for wire-bonding applications and are widely used, there is nevertheless a desire to further improve the performance of the transducer 100, such as by improving the oscillatory efficiency and amplification characteristics of the transducer at the capillary 106.